From German published application DE-OS 23 62 440 a device for opening a binding element is already known. This device is comprised of a flat element of a length greater than the length of the binding element and of a width greater than the diameter of the ring elements when closed. To facilitate the introduction of the device into the binding element, the device is tapered in the direction of insertion. By reason of the above-described dimensioning of the device, the ring elements of the binding element are bent open following insertion of the device in the binding element, their cross-section then having the form of an upwardly open ellipse. Then the perforations of a stack of sheets are placed onto the free ends of the ring elements. The ring elements being of a resilient configuration, they return to their initial position as soon as the device is withdrawn from the binding element. The individual sheets of the stack are held together by means of the binding element.
A further embodiment of a tool for opening a binding element is disclosed in European Patent EP 0 395 873 B1. This tool is equally of a flat configuration and has a leading section of a length at least the length of the binding element to be opened, with the width of the leading section being smaller than the diameter of the closed ring elements of the binding element. Adjoining the leading section in opposition to the direction of insertion is a succeeding section of a length again at least equaling the length of the binding element to be opened, yet of a width greater than the diameter of the ring elements when closed. Provided between these two sections is a further section which in the direction of insertion is as wide as the leading section while widening progressively in opposition to the direction of insertion until its width equals the width of the succeeding section. To open or close the binding element, the tool is pulled axially through the ring elements of the binding element.
Both known elements have the disadvantage that a flat tool automatically comes to lie in the plane of the large main axis of the ellipse when in the interior of the opened elliptical binding element. As a consequence, the length of the free ends of the ring elements is limited to the portion lying above the main axis. Therefore, with the format of the binding element given, the tool is only suited to bind relatively thin stacks of perforated sheets.
In addition, the tool described in European Patent EP 0 395 873 is very awkward to handle because of its dimensions, its length being at least double the length of the binding element employed.